1. Field of the Invention
The invention is directed to an apparatus for generating brief current pulses with high edge steepness and high final value for inductive users, particularly for gradient coils for nuclear magnetic resonance tomography, whereby a high user voltage is applied for fast build-up of a pulse and a low user voltage is applied for the maintenance thereof.
2. Description of the Prior Art
Particularly in imaging methods in medical diagnostics, magnetic fields, in which a patient is disposed, that are topically variable in all three spatial directions are super-imposed on a constant fundamental magnetic field and a radio-frequency alternating field, for three-dimensional resolution of the nuclear magnetic resonance signal picked up from the examination subject. These topically variable magnetic fields are also referred to as gradient fields and have a magnetic field strength that increases as linearly as possible in the respective direction. They are a prerequisite for the reconstruction of a subject image and are preferably generated by radio-frequency pulsing gradient coils, resulting in high demands made of the power supplies required for feed of the gradient coils. Such supplies must generate pulse-like currents having final values of several 100 A with rise times that lie in the millisecond range.
Known arrangements of this type are composed, for example, of four power transistors arranged in a bridge, having a shunt arm in which the inductive user is connected. In one embodiment, these transistors are operated as a linear governor or regulator having a range of adjustment of about half the overall supply voltage. The large losses thereby resulting are further increased because, due to the low current loadability, each of the four power transistors of the bridge is composed of about 60 real transistors. In another embodiment, the transistors are driven by a radio-frequency, pulse-shaped signal, so that the behavior of this power supply corresponds to that of a switched power pack. A disadvantage of this embodiment, however, is a user current relatively rich in upper harmonics.
German OS No. 31 12 280 discloses a further apparatus of this type wherein a high, constant voltage is first applied to the inductive user for the build-up of a pulse-shaped current having an arbitrary operational sign. After a desired pulse height is reached, a switch to a lower supply voltage is undertaken, this being adequate for the maintenance thereof, and the strength of current is kept constant by a regulating means by varying the voltage acting on the user. Free-running circuits serve the purposes of dismantling the respective curreint pulse, these free-running circuits being in part closed by engaging additional electronic rectifiers and, under certain conditions, enabling feedback of the energy contained in the inductive user into one of the two voltage sources.